Diagnosis: Metastatic calcification from chronic renal insufficiency

Full history:

This is a 53 year old woman with back pain, worsening over the last three months. She had bilateral lung transplantation three years prior to this examination secondary to severe chronic obstructive pulmonary disease. She has since been on immunosuppression and steroid therapy. Vertebroplasty to the lower thoracic spine was performed a year following her lung transplant secondary to compression fractures.

Radiopharmaceutical:

Findings:

There is mild uptake in T6 and moderate uptake in T8 consistent with subacute compression fractures. Mild uptake right lower rib is likely due to trauma.

Thoracic spine radiographs: There are vertebroplasties at T10, T11 and T12. Compression deformities at T6, T8 and L1 are noted, unchanged from comparison study 1 month prior (images not shown). There is atherosclerotic disease of the aorta.

Discussion:

Soft tissue accumulation of Tc99m MDP may be due to passive localization of tracer in slow fluid spaces (e.g. from ascities or pleural effusion) when the blood concentration of tracer is high. Subsequently, these spaces do not clear as rapidly as the blood pool.

Soft tissue calcification is another cause of soft tissue uptake of Tc99m MDP. Tracer may also bind in necrotic tissues (e.g. myositis or myonecrosis).

Causes of "metastatic calcification" include hypercalcemia from widespread destruction of bone (from metastatic cancer), chronic renal disease, primary parathyroid neoplasm and hypervitaminosis D. When the solubility product for calcium and phosphate is exceeded, there is precipitation of calcium in the extracellular space.

Acid secreting organs and organs in which there are rapid pH changes(i.e. the stomach, kidneys and lungs) can cause precipitation of calcium salts. This is well demonstrated in this case. Note however the absence of gastric uptake. This absence is presumed due to alterations in pH because the patient has been taking the H2 blocker, Prevacid. Myocardial calcification can also occur, which can be clinically significant especially when involving the cardiac conducting system.

Followup:

The patient is currently still on immunosuppression from her lung transplantion. She is followed closely for her chronic renal insufficiency and thoracolumbar compression fractures, which have remained stable to date. Her serum calcium has normalized after starting calcium carbonate supplimentation.

Differential Diagnosis List

If the patient had a recent lung scan, there may be residual activity in the lungs from Tc99m macroaggregated albumin or Tc99m DTPA aerosol. Primary hyperparathyroidism may demonstrate soft tissue accumulation of tracer in sites of metastatic calcifications such as the lungs, kidneys and stomach. Metastatic disease such as from osteosarcoma, breast or mucinous producing neoplasms may be considered. Trauma, rhabdomyolysis and infarction can demonstrate soft tissue uptake, but the distribution in the lungs and kidneys in this case is unusual.